1. Field of the Invention
The present invention pertains generally to measuring the thickness of the myocardial wall in a heart, and more particularly to a single-crystal ultrasonic sonomicrometer for measurement of absolute myocardial wall thickness.
2. Description of the Background Art
Measurement of myocardial wall thickness, as well as end-systolic and end-diastolic dimensions, are important in evaluating the effects of changes in regional myocardial function and contractility, including evaluating myocardial oxygen supply and demand, in acute and chronic animal studies. Presently, two types of commercially available instruments exist that utilize ultrasound to measure myocardial wall thickness in real-time. These include the transit-time sonomicrometry system and more recently, the Doppler echo displacement system.
A transit-time sonomicrometer uses two crystals, one as a transmitter and the other as a receiver, and operates by measuring the time required for ultrasound to travel between the transmitting and receiving transducers. An advantage of this system is its ability to provide an absolute dimension signal output calibrated in units of distance. However, the system has several disadvantages such as (1) it is necessary to insert a transducer through the myocardium, which can damage the myocardial nerve and blood vessel supply of the myocardial wall, (2) it is difficult to position precisely the endocardial crystal at the tissue/blood subendocardial interface, and (3) it can be difficult to maintain good alignment at all times throughout the cardiac cycle for short term and particularly during longer duration studies (&gt;12 weeks).
The Doppler echo displacement system addresses the necessity to avoid damage to the myocardium by using a single epicardial ultrasonic transducer. A non-tracking system was developed initially, while a tracking system has been developed more recently. The operating principles for both instruments are similar and are analogous to the operating principles of a blood-flow velocity meter.
In a non-tracking Doppler echo displacement system, a short burst of ultrasound pulses is transmitted from a single transducer sutured to the epicardium. After the transmission burst ends, the receiver circuits are enabled and the system receives echoes returning from underlying myocardial layers, using the same transducer. Phase detectors then measure the quadrature phase of the returned echoes that pass through a static sample volume. Integrating the phase zero crossings throughout the cardiac cycle provides a measurement of the velocity of myocardial layers passing through the sample volume. In theory, assuming that the velocity of the endocardium is changing linearly with respect to the velocity of the epicardium, a good estimate of myocardial thickening is expected. However, to avoid interference from ventricular chamber blood flow, the depth to which the sample volume can be advanced is limited to 1-2 mm less than the minimum end-diastolic excursion. An advantage of this system is the absence of damage to myocardium by the single epicardial transducer; however, the disadvantages are: (1) the instrument provides only an estimate of displacement rather than a measurement of absolute myocardial wall thickness, (2) its sample volume must be placed in a fixed location 1-2 mm less than the minimum end-diastolic excursion, a requirement that does not allow measurement of function from the entire subendocardium, the area most vulnerable to ischemia, and (3) an external reset mechanism (e.g. ECG) must be used to avoid baseline wandering, a requirement that imposes a preselected minimum end-diastolic dimension, a value that normally varies during the course of an experiment.
The tracking Doppler echo displacement system was subsequently developed to address deficiencies of the non-tracking version. The system function is nearly identical to that of the non-tracking system with the exception of a feedback circuit that is used to adjust the position of the sample volume. However, the tracking system does not employ a method to ensure that it reliably tracks the endocardial wall; rather, during the cardiac cycle, the feedback circuit adjusts the sample volume position in response to the integrated velocity profile of echoes passing through the sample volume. Like the non-tracking system, the tracking Doppler displacement system uses a synchronization signal, such as the R-wave of an ECG, to reset the tracking gate back to a predetermined end-diastolic position, set at the beginning of the experiment. The echoes reflected back to the epicardial transducer, and detected in the sample volume, usually are not from a single myocardial layer but rather from multiple reflectors. Since there is only one reference point throughout the R-wave synchronized cardiac cycle, however, it is possible to lose track of echoes reflected from the myocardial layer selected during R-wave synchronization, and follow a different myocardial layer. The system is designed to track all echoes that pass through the sample volume at the time of the synchronization pulse. This feature results in an end-diastolic dimension that does not change from beat-to-beat unless the user repositions the depth of the sample volume and recalibrates. The synchronization pulse resets the sample volume to the pre-selected position, regardless of its true position, which may have changed during an intervention. This system requires the user to frequently reposition the end-diastolic range of the instrument and to recalibrate for proper operation. Advantages of the tracking Doppler echo displacement system are: (1) the system only requires one transducer, placed on the epicardial surface, and (2) the output signal can be calibrated in absolute units of distance. However, its disadvantages are: (1) the system does not track the endocardial surface, so the measurement of myocardial wall thickness dimension may not represent actual wall excursion, and (2) the system is unable to continuously measure real-time changes in end-diastolic dimensions throughout interventions, without readjustment and recalibration.
Therefore, there is a need for an apparatus which can measure myocardial wall thickness in absolute units of distance between the entire epicardium and endocardium throughout the cardiac cycle using a single epicardial transducer, which operates without requiring frequent recalibration or other adjustments, and which is easy to use and calibrate. The present invention satisfies those needs, as well as others, and overcomes the deficiencies in the myocardial wall thickness measurement instruments heretofore developed.